I had a lot of problem to get this working, and for a while I was missing all the audio action on Youtube (too bad!) — so, here’s the solution that worked for me – Install “flashplugin-nonfree-extrasound”. It could be as easy as typing the following in a terminal and hitting ENTER:

sudo apt-get install flashplugin-nonfree-extrasound

UPDATE: To get Skype to work, verify that sound is working, and then open Skype / Options / Sound Devices, and update the settings as follows: